Experimental Study of Corrosion in Cables and Development of S-Parameter-Based Non-destructive and In-situ Monitoring Technology

Abstract

Cables are a key part of electrical and electronics systems, responsible for carrying electricity and signals over long distances. Ensuring their safety and reliability is essential to ensure electricity and signals are delivered without interruptions. Silver-plated copper cables are widely utilized in various high-performance applications by NASA, DOE and DOD due to better electrical and thermal conductivity, higher corrosion resistance, solderability, crimp ability, flexibility and durability compared to pure copper. However, silver-plated copper cables are highly susceptible to a specific kind of corrosion called red plague which significantly affects its mechanical and electrical properties including strength, ductility, fatigue life and electrical conductivity. Addressing red plague is a significant challenge for many NASA, DOE, and DOD systems. Thus far, all experimental studies concerning red plague have been conducted exclusively on cable samples without electrical current applied. However, in real-world applications—ranging from industrial environments to advanced systems used by NASA and the DOD—these cables are typically connected via solder joints, routinely carry electrical current, and operate for extended periods under varying atmospheric conditions. As regards the current direction in-plane to the copper-silver interface, it is well understood that the current itself will not affect the corrosion. Therefore, by best knowledge, there is no scientific finding/report on the corrosion in cables carrying DC current and the impact of current on corrosion. This approach aligns with current scientific knowledge, which suggests that the rate of corrosion growth should not be affected by the electrical current itself in the cables. As a result, investigating the specific impact of direct current (DC) on corrosion in silver-copper cables under real-world conditions is both urgent and essential. Currently, corrosion detection and monitoring techniques for corrosion under coating and insulation such as red plague are mainly destructive which involves peeling of the insulation layer for visual inspection of cables resulting in material damage, cable wastage, and significant corrosion-related costs. This is because detecting red plague, which occurs specifically in silver plated copper cables, presents unique challenges as the corrosion develops internally at the interface between the silver plating and the copper core, making it difficult to identify using conventional non-destructive testing methods. Therefore, it is important to develop a non destructive corrosion detection and monitoring technology to minimize catastrophic system failures and reduce the high costs associated with corrosion. In this work, red plague in silver-plated copper cables with and without DC current was experimentally studied and characterized under 90oF and 90% relative humidity atmospheric condition using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS) and nano x-ray CT techniques. Firstly, the corrosion rates/depth in cables with and without DC current including longitudinal and transversal was experimentally determined. The atmospheric spread/depth of corrosion for long term periods was predicted using corrosion models. The influence of DC current on the corrosion growth in cables was also experimentally exploited. The study revealed that DC current significantly accelerates corrosion, causing red plague to occur earlier, grow faster, and cover a larger area. Corrosion was also observed to spread along the cables in the direction of DC current. In addition, corrosion at the current input end (positive) was also found to be more severe than output end (negative) of the cables. Corrosion also initiates earlier and progresses faster in cables carrying higher current values or subjected to higher current densities. Furthermore, this research determined that the corrosion acceleration observed in cables with DC can be attributed to self-hall effect phenomenon. In silver-plated copper cables with DC current, the self-hall effect is believed to cause the electrons to drift away from the cable surface under the influence of Lorentz force, creating localized regions of electron depletion promoting anodic reactions and making the regions more susceptible to corrosion. Moreover, the Hall effect phenomenon in copper conductors was experimentally verified in this study, further supporting these findings. Finally, a novel time-dependent s-parameter-based non-destructive and in-situ technology was developed to evaluate the corrosion status in the silver-plated copper cables. Two techniques were developed to effectively represent the corrosion status in the cables, namely the loss function and peak analysis method. The loss function approach involves utilizing loss function formulas, specifically mean squared value, to compute the numerical difference differences of the s parameter readings at various time points. The peak analysis method involves using a Fast Fourier Transform smoothing and counting zero crossing using softwares for counting the number of peaks within specific frequency ranges. Ultimately, this dual approach – integrating experimental study and non-destructive technology – will not only provide a way to assess the operational readiness of the system in which silver-plated copper cables are used but would facilitate a substantial reduction in system failures and associated costs due to corrosion.